Hematopoiesis is sustained by a population of stem cells that can both replicate and differentiate. Pluripotent hematopoietic stem cells are the most primitive stem cells and are central to hematopoietic development. These cells can divide to make daughter cells or can differentiate through a hierarchial pathway of increasingly committed stem cells to mature blood cells. The ability of each stem cell to proliferate and differentiate is termed its developmental potential. During fetal development hematopoietic stem cells migrate from the yolk sac to fetal liver and ultimately to the bone marrow. The hematopoietic progeny of these cells colonize multiple organs during fetal development including spleen, liver, thymus, lung and brain. There is considerable evidence to suggest that the biological properties of hematopoietic cells resident in the yolk sac or fetal liver may differ from those in adult animals. Some but not all previous studies have suggested that the proliferative and self-renewal capacity of fetal hematopoietic stem cells resident in the liver are superior to those in the medullary cavity of the adult animal. However, due to the inability to identify stem cells or the progeny of individual stem cells, it has not been possible to characterize the developmental potential of individual hematopoietic stem cell clones during normal fetal development. The general goals of this proposal are: 1) To define the developmental potential of fetal hematopoietic stem cells resident in the fetal liver; 2) To evaluate the cellular, organ and clonal distribution of the hematopoietic progeny of these cells in selected tissues of neonatal and adult animals. We will achieve these goals by introducing foreign genes in utero into hematopoietic stem cells of rat fetuses by using retroviral vectors. Hematopoietic stem cells are actively proliferating, and are resident in high concentrations in the liver of midgestation rats, and are thus an attractive target for retroviral gene transfer. We have previously demonstrated that primitive hematopoietic precursors can be retrovirally transduced by direct injection of provirus in utero into the liver of midgestation rats. This unique approach to transducing hematopoietic cells in vivo will enhance the study of fetal hematopoietic development and may provide information relative to the potential correction of genetic diseases affecting hematopoietic cells and selected metabolic disorders in utero.